Why is it more difficult to do a sit up with your hands behind your head than when your arms are stretched out in front of you?

Imagine that someone pushes one marble toward a motionless marble. would there still be action-reaction forces involved in the collision? how might the marbles’ motions be changed? ?

If a balloon is taken outside on a very cold day, what will occur?

Arowboat passenger uses an oar to push the boat off the dock by exerting a force of 40n for 3.0s. what impulse acts on the boat.